Photoelectric relay



vMarch 4, 1941'. F, H, SHEPARD, JR 2,234,011

PHoToELEcTRIc RELAY Filed May 27, 1959 Snventor azz'sjevar Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE PHOTOELECTRIC RELAY Francis H. Shepard, Jr., Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to relay systems, and more particularly to an automatic photoelectric relay for controlling the operation of a device in response to a sudden predetermined percentage decrease of light intensity, regardless of the actual light intensity.

Photoelectric relays have been widely used to detect a change in a given condition or to start the operation of responsive mechanisms. One diculty which is commonly encountered is that which arises from changes in the light intensity falling on the photo tube, due, for example, to changes in the general illumination in the vicinity of the photoelectric tube or to line voltage variations causing changes in the light intensity of the lamp or other artificial light source which is used. The more sensitive the device is made, the more serious this limitation becomes. The more sensitive photoelectric systems are also frequently disturbed by small changes within the amplier tubes themselves, either due to the aging of the tubes or changing contact potentials, etc. As a result, the stable operation of highly sensitive photoelectric devices is difficult to achieve.

It is, therefore, an object of this invention to provide a highly sensitive photoelectric relay which is sensitive in any desired degree to sudden fluctuations in a given light source, but which is unaffected by slow changes.

It is also a purpose of this invention to provide a photoelectric relay which will operate logarithmically, that is, it operates when the light intensity falling on the sensitive photo tube decreases by :a predetermined percentage, regardless 'J of the actual values of the light intensity.

An additional object is to provide an instrument which will be sensitive only to a decrease in the received light intensity, and this even though the duration of the light is very brief. A twenty- 4 ve percent decrease, for example, should operate the relay. To accomplish this, it is necessary that no transients be set up by sudden increases in light intensity.

A further requirement which it is the object oi 45 this invention to ll relates to the ability of a relay to respond to impulses which occur over a range of frequency from four or ve cycles per second to one cycle in approximately thirty seconds or more. This must be accomplished simply and inexpensively, and the controlled device must not be operated by normal power supply voltage variations occurring within the desired frequency range.

This invention will be better understood from the following description when considered in connection with the accompanying drawing, in which Figs. 1 to 3 are curves illustrating the operation of this invention; Fig. 4 is a simplified form of my invention; and Fig. 5 is the schematic diagram of a preferred embodiment of my invention.

Referring to Fig. 1, a curve is illustrated in which the grid current flowing in the grid circuit of a vacuum tube is plotted as a function of the grid voltage. The shape of this curve is logarithmic, that is, equal increments of grid voltage correspond to a given percentage increase or decrease in the grid current, as the case may be. The logarithmic relation is due to the logarithmic distribution of electrons leaving the cathode as the grid voltage is varied. If the operation of the photoelectric relay can be made to conform to a curve of this type, it will be appreciated that the desired response characteristic will be achieved, since a given percentage decrease in the grid current produces the same incremental change in the grid Voltage regardless of theactual value of the grid current. A device which fulfills this requirement is illustrated in Fig. 4 to which reference is now made.

The device consists of a photosensitive section and an amplifier section. To aiford maximum economy, both sections are preferably operated directly from the commercial power lines without rectifiers and iilters.

A plug 5 is adapted to connect lines 1 and 9 to respective power lines which are not illustrated. Heater current for the filaments II, I3 of the tubes which :are used is obtained by a serially connected line voltage dropping resistor I5.

The anode I1 of a photoelectric tube I9 is connected to power line 1. The photo cathode 2I is connected to the grid 2'3 of a triode 25, the anode 21 of which is connected to power line 1. Cathode 29 is connected through a variable resistor 3I and a by-pass capacitor 33 to power line 9. The common point of resistor 3| and capacitor 33 is connected through a resistor 35 of high impedance to the anode electrode 31 of a thermionic rectifier 39. The anode is by-passed to power line 9 by means of capacitor 4I. The cathode 43 of tube 39 is connected to power line 1. Output is obtained between power line 9 and the common point of resistors 3| and 35 through a capacitor 45. The output voltage is amplied to any desired degree by means of an amplifier 41, the output of which is utilized to operate a relay 49.

v Several unique features are involved in the apparatus described above. In the first place, the use of a photoelectric tube as a grid return of the tube 25 permits logarithmic operation of the tube. The random flow of negative electrons to grid 23 tends to place a negative D. C. potential on the grid. The amplitude of the grid potential is determined by a balance between the velocity and number of the electrons and the amplitude of the current of the photo tube, which, in turn, is determined by the light intensity. Thus, variations in the photo current cause variations in the grid voltage, which control the anode current in the usual manner. The tube operation, therefore, corresponds to the logarithmic curve shown in Fig. l. A given percentage change in the light intensity produces a given change in the anode current throughout a Wide range.

The output is taken from the load resistor 35 through a coupling capacitor 45. The input to the amplifier 41 is, therefore, independent of the steady potential of the cathode, but any changes which occur at a rate above a predetermined minimum, 25% in 30 seconds, for example, are impressed on the amplifier. As pointed out above, the change in the cathode potential is the same for a given percentage change in photo current, or light intensity, throughout the operating range. Therefore, an impulse of a given amplitude is impressed on the amplifier for a given percentage change in the light intensity, assuming that the changes are accomplished at a constant rate. Changes which occur at a very slow rate, however, do not impress a sufficient voltage on the amplifier to operate the relay. The device is, therefore, insensitive to slow changes which result from the aging of the tubes, decrease in emission of the light source, and the like, but it will respond equally well to a 25% decrease in the light from a match held, say, 10 feet from the phototube, or to a 25% decrease in the light rcm a brilliant bulb held 1 inch from the photo The tubes of the amplifier 41 are preferably biased so that the applied negative voltage impulse which is produced by a sudden decrease in light intensity produces a positive impulse which is of sufficient amplitude to cause anode current to flow in the biased output tube, so that the relay is operated. An impulse of opposite polarity, produced by an increase in light intensity, merely increases the negative grid bias of the output tube of the amplifier and consequently does not actuate the relay. The device is therefore responsive only to a decrease of light intensity as desired. The use of a direct current amplifier will avoid transients which might otherwise cause the relay to operate on severe overloads.

In order to reduce to a minimum the effects of line voltage or power supply surges which occur Within the frequency band of the amplifier response, a novel bridge arrangement is used which, unlike ordinary bridges or balanced circuits, can be balanced over a substantial area of the power supply voltage-output voltage characteristic of the circuit. This object is accomplished by means of the circuit connections which include rectifier 39, the operation of which will be explained hereinafter.

Referring to Fig. 4, the effective D. C. voltage from line to cathode 29, and hence the output voltage across capacitor 33, is determined by two factors. The first is the positive line to cathode voltage produced by rectification within tube 25, and the second is the negative line to cathode voltage which is produced by rectification within the diode 39.

Neglecting for the moment the effect of diode 39 and remembering that both power lines 1 and 9 are at the same average D. C. potential, during alternate half cycles of the power line frequency a positive voltage with respect to the cathode is applied to anode 21, and current flows from line 1 to line 9 through the path which includes resistors 3l, 35 and capacitors 33 and 4I. This places a steady positive voltage with respect to the average line potential across capacitor 33 which is a filter to remove much of the A. C. voltage component. The upper portion of Fig. 2 illustrates a family of percent-line-voltage vs. line-to-cathode voltage curves produced in this manner corresponding to different Values of grid bias. In each case the line-to-cathode voltage becomes more positive as the line voltage increases.

Neglecting now for a moment the cathode voltage which is produced in the manner described immediately above, and assuming that the line to anode 21 voltage is Zero, consider the effect of the diode rectifier 39. The line voltage is applied across the diode rectifier and a negative D. C. potential with respect to the average line potential is developed between the diode anode 31 and line, which is impressed across capacitor 33 by resistor 35.

Y The lower portion of Fig. 2 is a family of curves corresponding to percent-line-voltage vs. D. C. line-to-cathode Voltage, plotted, as before, for various values of grid-to-cathode bias. In each case, the line-to-cathode voltage becomes more negative as the line voltage increases. Note that in this case the slopes of the curves are opposite to the Slopes of the curves referred to above.

Considering the combined effect of the cathode voltage developed by tube 25 and the superimposed voltage developed by the diode, illustrated in Fig. 2, it is seen that changes in the line voltage produce opposite effects. By adjusting the amplitudes of the two opposite effects complete compensation or any desired degree of overcompensation is achieved, so that changes in line voltage over a wide range produce no net voltage change on the cathode of tube 25, and consequently no change of output. Fig. 3 shows a family of curves in which complete compensation has been achieved. The anode current of the tube is then controlled only by the cathodeto-grd bias voltage, which is a function of the photoelectric current. 'Die output voltage is, therefore, a function only of changes in light intensity and is independent of changes in the line voltage.

It is also to be noted that the balanced or compensated condition exists throughout wide ranges of grid voltage. The system is balanced not only for a particular condition, but is balanced throughout an area bounding the linear horizontal portion of the composite family of curves obtained by the combination of the two families of curves which were described above, as indicated by the dotted line in Fig. 3.

While this invention is adapted to be used in conjunction with any suitable amplifier, in a preferred embodiment illustrated in Fig. 3, it is proposed to utilize an A. C. operated D. C. amplifier 59 of the type described and claimed in my application Serial No. 727,968, filed May 28, 1934, which issued November 22, 1938 as U. S. Patent 2,137,419. The particular modification herein illustrated is described and claimed in my copending application Serial No. 269,921, filed April 25, 1939, and assigned to the assignee of the present application.

Referring to Fig. 5, the circuit arrangementA from line plug to the amplier input is substantially as shown in F'ig. 4, and since similar parts bear similar reference numerals it need not be described again. The only changes involve the addition of a low-pass lter in the D. C'. output lead, comprising resistor 5I and capacitor 53, and the use of a diode-triode tube 55 as the first amplifier tube so that the separate diode rectifier 39 of Fig. 4 is no longer necessary.

A rectified compensating bias voltage is impressed on the cathode of tube 25 by the action of the diode section of the rst amplifier tube 55. A grid resistor 51 is connected between the grid of the triode section of the rst amplifier tube and line 9. The amplier circuit illustrated in Fig. 5 is especially suitable for use in connection with the present invention due to its simplicity and stability, and due to the fact that no rectiiier-lter power supply system is required. By utilizing the diode-triode tube combination illustrated the device is further simplified.

I have thus described a photoelectric relay which is highly sensitive, stable, economical, free from diiiiculties due to line voltage variations, and which responds to a desired percentage change in light intensity Without regard to the actual intensity of the light received by the phototube.

I claim as my invention;

l. A photoelectric relay comprising a thermicnic tube having input and output electrodes, light responsive means connected to said input electrodes for controlling the potential of said input electrode, means for applying an alternating energizing voltage to said output electrode, said energizing voltage producing a direct output voltage whose amplitude is a combined function of the potential of said input electrode and the amplitude of said energizing voltage, and rectifying means for compensating for changes in said output voltage due to changes in the amplitude of said energizing voltage.

2. A photoelectric relay comprising a thermionic tube having input and output electrodes, light responsive means connected to said input electrode for controlling the potential of said input electrode, alternating means for applying an energizing voltage to said output electrode, said voltage producing a rectified output voltage, means including a rectifier for compensating for changes in the amplitude of said output voltage due to changes in the amplitude of said energizing voltage, and means operable in response to changes in the amplitude of said output voltage.

3. A photoelectric relay comprising a thermicnic tube having input and output electrodes, light responsive means connected to said input electrodes for controlling the potential of said input electrode, means for applying an alternating energizing voltage to said output electrode, said alternating voltage producing a rectified output voltage the amplitude of which changes in a given direction as a function of the peak amplitude of .said alternating energizing voltage, means for producing a second rectified voltage the amplitude of which changes in the opposite direction as a function of the peak amplitude of said alternating energizing voltage, means including said second rectiiied voltage for compensating changes in the amplitude of said output voltage whereby the output voltage of said thermionic tube is determined by the intensity of said light and is independent of changes in the amplitude of said energizing voltage.

A4. A photoelectrc relay of the character described in claim 3 which includes in addition indicating means responsive to changes in said rectified Voltage.

5. An alternating current operated photoelectric relay Which includes a thermionic tube having cathode, grid, and anode electrodes, light responsive means connected to said grid electrode for Varying the potential of said grid electrode, means for applying an alternating energizing voltage to said anode electrode, said alternating voltage producing a positive output voltage the amplitude of which varies directly as the peak amplitude of said alternating voltage, means for producing a negative voltage the amplitude of which varies directly as the peak amplitude of said alternating voltage, and means for combining said negative voltage and said output voltage to compensate for changes in said positive output voltage produced by changes in the peak amplitude of Asaid alternating energizing Voltage, whereby the resultant output voltage of said thermionic tube is determined by the intensity of the light falling on said light responsive means, and is independent of changes in the peak amplitude of said energizing current, and means operable in response to changes in the intensity of said output voltage.

6. A device of the character described in claim 5 in which said means for producing a negative voltage includes a rectifier energized by said alternating energizing voltage.

7. A device of the character described in claim 5 in which said means responsive to changes in the intensity of said output currents includes a relay.

8. A device of the character described comprising means for connecting a pair of conductors to a source of alternating voltage, a grid-controlled rectifier connected across said conductors and including means` for developing a steady potential of a predetermined po-larity with respect to the average potential of said conductors, a second rectier connected across said conductors and including means for developing a second steady potential of a polarity opposite to said predetermined polarity With respect to the average potential of said conductors, means for combining said steady voltages to produce a resultant potential, and control means connected to said grid-controlled rectier for varying the amplitude of said resultant potential independently of changes in the amplitude of said alternating voltage.

9. A device of the character described comprising means for connecting a pair of conductors to a source of alternating voltage, a grid-controlled rectiiier connected across said conductors and including an impedance across which a direct voltage is developed having a given polarity with respect to the average voltage of said conductors, a second rectier connected across said conductors and including an impedance across which a second direct voltage is developed having a polarity opposite to said given polarity, means for combining said direct voltages to produce a resultant output Voltage, and a photcelectric tube connected to said grid-controlled rectier for varying the amplitude of said resultant voltage as a function of the light intensity impinging on said tube independently of changes in the amplitude of said alternating voltage.

10. A device of the character described ccmprising means for connecting a pair of conductors to a source of alternating voltage, a thermionic tube having grid, cathode and anode elective impedance means, means for combining said voltages to produce a resultant voltage, and control means connected to said grid electrode for varying the amplitude of said resultant voltage independently of changes in the amplitude of said 5 alternating voltage.

FRANCIS H. SHEPARD. JR. 

